1. Technical Field of the Invention
The present invention relates to treads for tires designed for fitting on the front axle of vehicles and, more particularly, heavy vehicles, which may be required to travel long distances at sustained speeds.
2. The Related Art
Such tires generally have a carcass reinforcement comprising a plurality of reinforcing elements arranged radially, the carcass reinforcement itself being covered by a crown reinforcement that extends in the circumferential direction. This crown reinforcement is composed of at least two plies superimposed over one another, each ply being formed of a rubber mixture reinforced by a plurality of cords or wires with low extensibility, preferably made of steel, which are arranged parallel to one another in the same ply and are inclined in a direction of at most 40° relative to the circumferential direction, with the cords of each ply being crossed from one ply to the next. The crown reinforcement can be completed, on the one hand, by two half-plies formed of cords with low extensibility which make an angle between 45° and 80° relative to the circumferential direction and, on the other hand, by at least one ply formed of cords, said to be “elastic”, which are positioned radially outside the crown plies and whose reinforcing cords make an angle smaller than 40° relative to the circumferential direction.
In addition, the crown reinforcement is covered on its radially outer surface by a tread made of at least one rubber mixture, whose part radially on the outside of the tire forms the rolling surface intended to come into contact with the road during the rolling of said tire.
To obtain satisfactory grip when rolling on a road covered with water, the tread is provided on its external surface, in the case of tires designed for fitting on the front, steering axle of heavy goods vehicles, with a plurality of grooves having substantially circumferential orientation. These grooves form a pattern comprising a plurality of ribs, each rib having a contact surface radially on the outside and side walls that can be substantially perpendicular to the contact surface of the rib or that can make a non-zero taper angle with that surface (undercut geometry). The intersection of each side wall with the contact surface of a rib forms an edge.
During the rolling of tires provided with such treads wear has been observed which is termed “irregular” because it takes place to a greater extent at and close to the edges of the ribs than over the remainder of the contact surface of the ribs. This localized, irregular wear has several disadvantages: besides an unaesthetic appearance, it may necessitate a premature tire change; and such wear also impairs the proper mechanical function of the ribs when the tire is subjected to transverse forces, particularly when taking a bend, since the edges are displaced relative to the contact surface, which surface is consequently proportionally smaller.
To improve the resistance of such treads to irregular wear, U.S. Pat. No. 6,196,288 recommends that a plurality of incisions be made near each edge of each rib, the traces of these incisions on the contact surface being between 4 and 8 mm long and the incisions having a mean, constant inclination within the thickness of the rib of between 5° and 15° relative to a perpendicular to the contact surface.
Although such an arrangement is effective in combating this type of irregular wear, it is found that the mean overall wear of ribs provided with such incisions is increased very considerably compared with the overall wear of ribs without such incisions. Thus, although the wear of each rib becomes more regular, the wear life of the tread is shorter compared with that of a tire whose tread has no such inclined incisions. “Wear life” means the duration of possible use before the tread of a tire becomes so worn during rolling that either the tire must be changed or the tread must be renewed by recapping.
There is therefore a need for a tread structure for tires designed for fitting on the front axle of heavy vehicles, which tread structure includes at least one rib that shows little or no irregular wear and with which the wear rate for all the ribs is substantially the same while the average overall wear rate remains low. These characteristics confer on the tire a wear life that is improved compared with that of tires as disclosed in the aforementioned U.S. Pat. No. 6,196,288.